1. Field of the Invention
The present invention relates to a photoelectric encoder provided with a scale having a grating of a predetermined pitch formed thereon and a light source and a light receiving unit, which are relatively displaceable with respect to the scale, and in particular to a photoelectric encoder that is favorable in use of a linear encoder and is capable of reducing a position detection error resulting from stains on the scale and/or defects of the grating.
2. Description of the Related Art
Conventionally, a photoelectric encoder has been adopted for precise measurement of the amount of linear movement of a substance. As one of the position detecting methods in which an optical encoder is adopted, an arctangent calculation of Lissajous signals obtained from a photodiode array has been frequently used.
For example, in the method proposed in Japanese Published Unexamined Patent Application No. Sho64-57120 (hereinafter called Patent Document 1), as shown in FIGS. 1 and 2 thereof, a light-receiving element composed of a P-type semiconductor layer 34 forming a photosensitive zone 35 is first arranged in an array state at a disposing pitch by which a phase difference is brought about with respect to the phase of the optical grating 12 of the scale 13. Next, Lissajous signals are generated by differentially amplifying the output signals of the light-receiving element by differential amplifiers 38A and 38B, and further, the position is detected by carrying out an arctangent calculation with respect to the Lissajous signals.
However, as shown in FIG. 1 of the present application, where stains are adhered to the scale and/or where the grating of the scale has a defect, the output signals of the light-receiving element 34 are brought into imbalance since the sizes of the stains and/or grating defects are varied and the measurement light irradiated onto the light-receiving element 34 is interrupted. As the output signals of the light-receiving element 34 are brought into imbalance, the direct current component of the Lissajous signal is offset from the normal position shown by a solid line in FIG. 2 to the position shown by a broken line, wherein there is a problem by which an error is brought about in the position detection based on the arctangent calculation.